1. Field of the Invention
This invention relates to electromagnetically operated shutters and more particularly to improvements of a drive circuit for such shutter.
2. Description of the Prior Art
The conventional electromagnetically operated shutters have a drive circuit as shown in FIG. 1. In FIG. 1, 1 and 2 are one-shot circuits. Responsive to a leading curtain actuating signal, the first one-shot circuit 1 produces a drive control pulse of a predetermined width. Similarly, the second one-shot circuit 2 responsive to a leading curtain returning signal (from a circuit (not shown) delayed a certain time from the leading curtain actuating signal) produces a second drive control pulse of a predetermined width. Transistors 3, 4, 5 and 6 are connected to each other and to a magnetic winding 7 so that when in order to open the shutter or to initiate an exposure, the actuating signal is applied to the first one-shot circuit 1 with the production of the leading curtain drive control pulse as shown on line (1) in FIG. 2 at an output line, a, which is then applied to the transistors 3 and 6 at their bases. In this manner the transistors 3 and 6 are turned on to supply the magnetic winding 7 with current flowing in a forward direction indicated by a solid line I1, since the transistors 4 and 5, at this time, remain off. Thus, the leading curtain is driven to run down, initiating an exposure, by this electromagnetic drive source. After the termination of the exposure, the leading curtain returning signal is applied to the second one-shot circuit 2 which then produces the drive control pulse as shown on line (2) in FIG. 2. This pulse is applied to the transistors 4 and 5 at their bases, and transistors 4 and 5 are, thus, turned on. Since, at this time, the transistors 3 and 6 are already turned off, the magnetic winding 7 is supplied with current flowing in a direction indicated by a dashed line arrow I2, thus driving the leading curtain to return. Another drive circuit similar to that of FIG. 1 is provided for controlling the running down and returning operation of the trailing curtain of the shutter. The actuating signals for the leading and trailing curtain control circuits are provided in either electrically or mechanically timed relationship to obtain an exposure time.
FIG. 3 shows an example of modification of the drive circuit of FIG. 1 with addition of a circuit for preventing the leading curtain from bounding back from the terminal end of movement thereof. Elements 31, 32, 33 and 34 are one-shot circuits responsive to leading curtain running down, running brake, returning and returning brake signals respectively and produce drive pulses at their output lines, a to c. Elements 35, 36 are AND gates; 37, 38 are inverters; and 39, 40 are OR gates having outputs connected to the inputs of the transistorized portion of the circuit of FIG. 1. Elements 41 and 42 are switches as the sources of the brake signals. Though in FIG. 3 there is shown only the leading curtain drive circuit, a trailing curtain drive circuit of similar construction to the above is provided also. In operating the circuit, when, in order to initiate a running down of the leading curtain, an actuating signal is applied to the first one-shot circuit 31 which then produces a drive pulse as shown on line (1) in FIG. 4. This pulse is applied through the AND gate 35 and OR gate 39 to turn on the transistors 3 and 6. The magnetic winding 7 is thereby supplied with current flowing in the forward direction I'1. Thus, the leading curtain starts to run down. When the leading curtain nears the terminal end of movement thereof, the switch 41 is automatically closed to produce the brake signal which is then applied to the second one-shot circuit 32. The output pulse from the one-shot 32 is shown on line (2) in FIG. 4. This pulse, after having been inverted in polarity by the inverter 37 is applied to the AND gate 35, thereby the output of the AND gate 35 is changed to low level and held therein for a time t1 shown on line (2) of FIG. 4. Therefore, the output of OR gate 39 turns off the transistors 3 and 6, while the brake pulse from the one-shot circuit 32 after having passed through the OR gate 40 simultaneously turns on the transistors 4 and 5 for that time t1. As a result, the magnetic winding 7 is supplied with current flowing in the reversed direction I'2, thus stopping the leading curtain under the braking action at the terminal end of running down movement, and avoiding the bound of the leading curtain. Then, upon returning the leading curtain, a similar actuation of the one-shot circuits 33 and 34 to the above leads to the production of drive and brake pulses as shown on lines (3) and (4) in FIG. 4 which cause the supply of current flowing in the reversed direction I'2 to the magnetic winding 7 and therefore cause the returning movement of the leading curtain to terminate while preventing a bound.
Since such circuits of FIGS. 1 and 3 have to employ two transistors to supply the magnetic winding with the drive current from the battery, the voltage loss by the two transistors is so great that a battery of small capacity with low voltage such as that usually incorporated in the camera housing is not sufficient to assure the satisfactory operation of the shutter. Another disadvantage arising from the necessity of using four drive transistors in each of the leading and trailing curtain drive circuits is that the space which the eight transistors in total occupy is large. Further, since a great number of transistors must be used at a time with uniformity of characteristics thereof, it has been found that the conventional drive circuits described above are not amenable to mass production techniques.